Are you using the latest version of the theme and the latest version of WordPress? If you’ve upgraded WordPress but not the theme then it has been known to cause issues with older versions. You can download the latest version of the Restaurant theme from your Themeforest account. Then upload the theme using FTP to replace the existing version which is already there, this will keep your site running with all the settings as they currently are etc, as the page/post content is all saved to the WordPress Database.
I’m not sure what you’re referring to with the cache, any caching would be from either the host or a plugin you’ve installed – the theme doesn’t have any form of cache.
Hopefully reuploading the new version of the theme via ftp will resolve this issue for you.